The second directive is that ACLs seems to ignore this dynamic group:
# admins
access to dn.subtree="dc=msr-inria,dc=inria,dc=fr"
by group="cn=admins,ou=groups,dc=msr-inria,dc=inria,dc=fr" write
by * break

Try

access to dn.subtree="dc=msr-inria,dc=inria,dc=fr"
by
group/groupOfURLs/memberURL="cn=admins,ou=groups,dc=msr-inria,dc=inria,dc=fr"
write
by * break

(please excuse any unintended line wrapping).

Indeed, many thanks.

This is an often overlooked requirement of groups that use group
objectClass and member attribute other than groupOfNames and member.
Probably a note should be added to slapo-dynlist(5) man page and
somewhere else as appropriate.